This invention relates to an environmental control system which may be for use on board an aircraft.
More particularly the invention relates to an improvement in an environmental control system which is generally as described in EP-A-0738655. In the earlier proposal, an environmental control system is described which includes a cooling turbine connected in use, to a compressor, with there being a driving device for providing drive power to the cooling turbine-compressor combination, and an intercooler apparatus, wherein, in a first mode of operation, the air circulates around a closed cooling loop from the compressor to the intercooler apparatus to the cooling turbine, thence along a cooling pass to cool equipment or a volume, and back to the compressor, the system including a mode valve operable in a second mode of operation to open the loop to allow air to vent to ambient during or after the cooling pass whilst further working fluid is introduced into the loop.
Where the prior proposal is applied to an aircraft, when such a system is operating in the second, open loop, mode of operation, which will typically occur when the aircraft is on the ground, fresh bleed air from an auxiliary power unit (APU) or an aircraft engine is supplied both to a power turbine (which is a driving device which drives the cooling turbine-compressor combination) and to the open loop, i.e. to the inlet of the compressor, by way of a pressure regulating valve. Thus a large mass flow of air is required from the APU and/or engine for driving the power turbine and for providing air to the open cooling loop of the system, and effectively to cool such a large mass flow of air requires large pre-coolers and an increased supply of coolant air to the pre-coolers. Pre-coolers of increased size are not, of course, readily accommodated when space is limited e.g. on an aircraft.
According to one aspect of the present invention, we provide an environmental control system having a cooling loop including a cooling turbine which, in use, drives a compressor, and an intercooler apparatus, for cooling air compressed by the compressor prior to the air being expanded and cooled over the cooling turbine, and a heat exchanger to which at least a proportion of the cooled and expanded air from the turbine is fed to cool a heat load, and wherein, in a first mode of operation, the cooling loop is closed so that the air circulates around the closed loop from the compressor to the intercooler apparatus to the cooling turbine, and thence to the heat exchanger, and back to the compressor, and in a second mode of operation the cooling loop is open to allow air to pass from the system whilst further air is introduced into the cooling loop, and wherein an ambient air inlet valve is provided to provide for the admission of ambient air to the cooling loop, when the system is operating in the second mode of operation.
When the system is operating in the second operational mode, all of the high pressure air provided by the APU and/or engine may be used for purposes other than in the cooling loop, for example for providing drive to the cooling turbine-compressor combination in the cooling loop.
Usually a pressure regulating valve is provided between the cooling loop and a high pressure inlet for the high pressure air from the APU and/or engine, the pressure regulating valve permitting the introduction of high pressure air into the cooling loop when the pressure in the cooling loop is below a predetermined pressure e.g. to make up losses to the cooling loop when the system is operating in the first operational mode of operation.
Desirably, the ambient air inlet valve is provided downstream of the pressure regulating valve.
The cooling loop may be closed and opened by a mode valve in the cooling loop which may be provided upstream of the mode valve and downstream of the pressure regulating valve where provided.
Because of unavoidable cycle inefficiencies, the system may include a driving device to provide additional drive power to drive the compressor, such as a power turbine over which high pressure air is passed, the power turbine and the cooling turbine and the compressor being provided on a common shaft and being rotatable together. High pressure air for the power turbine may provided to the power turbine from upstream of the pressure regulating valve.
Cooled and expanded air from the power turbine may be fed to the intercooler apparatus to cool compressed air in the cooling loop, and the intercooler apparatus may include a ram air heat exchanger for cooling the compressed air.
Cooled and expanded air from the power turbine may additionally or alternatively be fed to a pre-cooling apparatus to pre-cool high pressure air from an inlet for high pressure air to the system and the pre-cooling apparatus may include a ram air heat exchanger.
According to a second aspect of the invention we provide an aircraft including a air conditioning system having a cooling loop including a cooling turbine which, in use, drives a compressor, and an intercooler apparatus, for cooling air compressed by the compressor prior to the air being expanded and cooled over the cooling turbine, and a heat exchanger to which at least a proportion of the cooled and expanded air from the turbine is fed to cool a heat load, and wherein, in a first mode of operation, the cooling loop is closed so that the air circulates around the closed loop from the compressor to the intercooler apparatus to the cooling turbine, and thence to the heat exchanger, and back to the compressor, and in a second mode of operation the cooling loop is open to allow air to pass from the system whilst further air is introduced into the cooling loop, and wherein an ambient air inlet valve is provided to provide for the admission of ambient air to the cooling loop, when the system is operating in the second mode of operation.